1. Field of the Invention
This invention relates to vehicular radar systems, and more particularly to a vehicular collision avoidance radar system which tracks a target within a narrowed radar beamwidth.
2. Description of the Prior Art
There is a continuing need to increase the density of vehicles traveling the world's roadways and simultaneously to improve the safety of highway vehicle operations by preventing highway vehicles from colliding with stationary and moving objects (such as roadside obstacles and other vehicles). One means for accomplishing these seemingly contradictory goals is to monitor the relative speed, direction of travel, and distance between vehicles sharing the roadway, and to use such information to provide direct indications to the vehicle's operator of potential danger. It is becoming increasingly more common for automotive engineers to consider the use of microwave radar systems as a means to monitor such environmental conditions.
Vehicle borne radar systems which transmit and receive at three different frequencies on a time division basis, with two of the frequencies being used to determine range, and the third being combined with one of the first two to determine closing speed and likelihood of collision, are presently known. One such system is disclosed in U.S. Pat. No. 3,952,303 to Watanabe et al., which teaches an analog radar system processing front end.
Another example of an automotive radar system is described in U.S. patent application, Ser. No. 07/695,951, entitled Multi-Frequency Automotive Radar System, and assigned to the assignee of the present invention. In that system, a transmit signal and the reflected received signal are coupled to an RF mixer. The relevant output from the RF mixer is a signal that has a frequency equal to the difference between the transmit and receive frequencies. The frequency of the reflected received signal may be shifted from the frequency of the transmit signal upon its return due to the "Doppler" effect. Doppler effect occurs whenever a transmitted signal reflects off a target that has a motion relative to a transceiver. The resulting frequency shift is referred to as "Doppler shift".
A further example of an automotive radar system, which uses principally a digital approach, is described in U.S. patent application, Ser. No. 07/930,066, entitled Multi-Frequency, Multi-Target Vehicular Radar System Using Digital Signal Processing, and assigned to the assignee of the present invention. In that system, which includes a transmit section for generating a two-channel transmit frequency, an antenna both transmits the transmit signal and receives a reflected receive signal. A diode mixer generates a difference signal having a frequency equal to the transmit frequency minus the received frequency. A signal switch in a front end electronics section time multiplexes and samples the channel one and channel two signals, following which the samples are coupled to a two-channel analog-to-digital converter. A digital electronic section receives the digital information and performs a Fast Fourier Transform on each channel of digital data to determine relative speed and range of a target based upon the frequency and the difference in phase of the two channels. The digital electronic section also receives information regarding the status of vehicle operation and/or controls to determine the degree of danger presented by an identified target.
In vehicular collision avoidance radar systems, it is frequently necessary to know at all times whether or not the target is at an off-boresight (i.e., off-axis) angle relative to a boresight (or reference azimuth), and if so the direction and amount of the angular error (angular deviation from the reference azimuth). It is also desirable or necessary to know the distance or range of the target. To provide for continuous tracking, a number of systems have been proposed including those which transmit a signal and then combine a multiplicity of diversely received replicas of the signal. Examples of such systems are provided by U.S. Pat. Nos. 4,060,809 of Baghdady, 4,975,710 of Baghdady, 5,084,709 of Baghdady, and 5,128,969 of Baghdady. However, such systems have proven to have shortcomings which make them undesirable or impractical when employed for use in vehicular collision avoidance radar systems. Such systems are often lacking in the accuracy by which target deviation from a reference azimuth is measured, and also by the difficulty or inability to define a limited tracking beam so as to be able to track a target within a confined angular range to the exclusion of other potential targets, such as vehicles off to the side of the vehicle carrying the radar system.
The present invention provides a system which limits the beamwidth of a radar system by distinguishing between targets that are at acute angles with respect to a reference azimuth, and those targets that are at obtuse angles with respect to the reference azimuth.